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trace: use a static buffer instead of tal_fmt().

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There's an EBPF limit anyway, so stick with a 512-byte buffer.

This brings us back to 621ns per trace:

Before:
	real	0m13.441000-14.592000(14.2686+/-0.43)s
	user	0m11.265000-12.289000(11.9626+/-0.37)s
	sys	0m2.175000-2.381000(2.3048+/-0.072)s

After:
	real	0m5.819000-6.472000(6.2064+/-0.26)s
	user	0m3.779000-4.101000(3.956+/-0.12)s
	sys	0m2.040000-2.431000(2.2496+/-0.15)s

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
This commit is contained in:
Rusty Russell
2025-04-23 10:24:40 +09:30
parent 3973c35013
commit b4dcf4e55d
1 changed files with 32 additions and 18 deletions
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50
common/trace.c
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@@ -257,11 +257,15 @@ static struct span *trace_span_slot(void)
return s;
}
#define MAX_BUF_SIZE 2048
static void trace_emit(struct span *s)
{
char span_id[HEX_SPAN_ID_SIZE];
char trace_id[HEX_TRACE_ID_SIZE];
char parent_span_id[HEX_SPAN_ID_SIZE];
char buffer[MAX_BUF_SIZE + 1];
size_t len;
/* If this is a remote span it's not up to us to emit it. Make
* this a no-op. `trace_span_end` will take care of cleaning
@@ -275,36 +279,46 @@ static void trace_emit(struct span *s)
if (s->parent)
hex_encode(s->parent_id, SPAN_ID_SIZE, parent_span_id, HEX_SPAN_ID_SIZE);
char *res = tal_fmt(
NULL,
"[{\"id\": \"%s\", \"name\": \"%s\", "
"\"timestamp\": %" PRIu64 ", \"duration\": %" PRIu64 ",",
span_id, s->name, s->start_time, s->end_time - s->start_time);
tal_append_fmt(&res, "\"localEndpoint\": { \"serviceName\": \"%s\"}, ",
trace_service_name);
len = snprintf(buffer, MAX_BUF_SIZE,
"[{\"id\":\"%s\",\"name\":\"%s\","
"\"timestamp\":%"PRIu64",\"duration\":%"PRIu64","
"\"localEndpoint\":{\"serviceName\":\"%s\"},",
span_id, s->name, s->start_time, s->end_time - s->start_time, trace_service_name);
if (s->parent != NULL) {
tal_append_fmt(&res, "\"parentId\": \"%s\",", parent_span_id);
len += snprintf(buffer + len, MAX_BUF_SIZE - len,
"\"parentId\":\"%s\",",
parent_span_id);
if (len > MAX_BUF_SIZE)
len = MAX_BUF_SIZE;
}
tal_append_fmt(&res, "\"tags\": {");
len += snprintf(buffer + len, MAX_BUF_SIZE - len,
"\"tags\":{");
if (len > MAX_BUF_SIZE)
len = MAX_BUF_SIZE;
for (size_t i = 0; i < SPAN_MAX_TAGS; i++) {
if (!s->tags[i].name)
continue;
tal_append_fmt(&res, "%s\"%s\": \"%.*s\"", i == 0 ? "" : ", ",
s->tags[i].name,
s->tags[i].valuelen,
s->tags[i].valuestr);
len += snprintf(buffer + len, MAX_BUF_SIZE - len,
"%s\"%s\":\"%.*s\"", i == 0 ? "" : ", ",
s->tags[i].name,
s->tags[i].valuelen,
s->tags[i].valuestr);
if (len > MAX_BUF_SIZE)
len = MAX_BUF_SIZE;
}
tal_append_fmt(&res, "}, \"traceId\": \"%s\"}]", trace_id);
DTRACE_PROBE2(lightningd, span_emit, span_id, res);
len += snprintf(buffer + len, MAX_BUF_SIZE - len,
"},\"traceId\":\"%s\"}]", trace_id);
if (len > MAX_BUF_SIZE)
len = MAX_BUF_SIZE;
buffer[len] = '\0';
DTRACE_PROBE2(lightningd, span_emit, span_id, buffer);
if (trace_to_file) {
fprintf(trace_to_file, "span_emit %s %s\n", span_id, res);
fprintf(trace_to_file, "span_emit %s %s\n", span_id, buffer);
fflush(trace_to_file);
}
tal_free(res);
}
/**